The present application is based upon and claims priority to Japanese Application No. 2001-217251, with a filing date of Jul. 17, 2001, which is hereby incorporated by reference.
A process for preparing polycarbonates and an apparatus for use in the process are disclosed herein.
Polycarbonates are widely used for their outstanding mechanical properties, such as impact resistance, heat resistance, and transparency. Processes known in the art for preparing polycarbonates include, for example, an interfacial process and a melt process. In the interfacial process, an aromatic diol, such as bisphenol A is directly reacted with phosgene. The melt process includes transesterification of an aromatic dihydroxy compound, such as bisphenol with a diallyl carbonate, such as diphenyl carbonate in the molten state. The transesterification is generally preferred, because it does not use phosgene, which can be toxic, or solvents such as methylene chloride. It is also allows more economic production.
The use of end-capping agents for end-capping polycarbonates polymerized by melt processes has been investigated, to with the goal of improving the hue, heat resistance, and hydrolysis resistance of the polycarbonates. For example, Japanese Patent Application Kokai No. Sho 63-179301 discloses a bisphenol A polycarbonate in which the ends are capped with methyl salicylate. Kokai No. Hei 2-175723 discloses a process for preparing polycarbonate in which the percentage of terminal hydroxyl groups is less than 30% and the limiting viscosity is 0.3 to 1.0 deciliters/gram (dL/g) (20xc2x0 C. in methylene chloride), by polycondensing an aromatic dihydroxy compound and carbonic acid diester in the presence of phenols having 10 to 40 carbon atoms, for example butylphenol, cumylphenol, and phenylphenol.
It should be noted, however, that even with the use of end-capping agents, the degree of polymerization will not be sufficiently high, and low-molecular-weight polycarbonate and/or polycarbonate with a broad molecular weight distribution will be obtained if the polycondensation reaction has not progressed sufficiently. Furthermore, end-capping efficiency may be poor even if the agent is added after the polycondensation reaction has progressed to the desired point. Thus, the time at which end-capping agent is added and mixing conditions are important factors.
For this reason, Kokai No. Hei 10-36497 discloses an improved process for preparing aromatic polycarbonates by melt polycondensation of an aromatic dihydroxy compound and diphenyl carbonate. An end-capped polycarbonate having an intrinsic viscosity varying no more than 0.1 dL/g is obtained by adding the compound shown in formula (1) below after. the intrinsic viscosity of the polycarbonate has reached at least 0.3 dL/g: 
wherein R1 is a chlorine atom, a methoxycarbonyl group, or an ethoxycarbonyl group; and R2 is an alkyl group having 1 to 30 carbon atoms, an alkoxy group having 1 to 30 carbon atoms, an aryl group having 6 to 30 carbon atoms, or an aryloxy group having 6 to 30 carbon atoms.
International Publication No. WO 98/47938 also proposes adding, and kneading for at least 0.1 seconds, an end-capping agent described by formula (1) above in proportions of 0.3 to 4.0 mol equivalents, based on the hydroxy terminals of the polycarbonate, under conditions of no more than 200-350xc2x0 C. temperature and no more than 1013 hektopascal (hPa) (760 millimeters of mercury (mmHg)) pressure and then adding, and kneading for at least 0.1 seconds, a stabilizer at a temperature of no more than 200 to 350xc2x0 C. and pressure of no more than 1.333xc3x97105 hPa (105 mmHg).
Furthermore, Kokai No. Hei 7-90074 discloses an improved process for preparing polycarbonates with an increased degree of polymerization by transesterification of a dihydroxy compound and a carbonic acid diester in which an active diester, acid halide, or acid anhydride having a valance of at least two is added after the transesterification rate has exceeded at least 70%. In Kokai No. Hei 6-1 57739, the present applicant also proposed an improved process for preparing aromatic polycarbonates by melt polycondensation of an aromatic dihydroxy compound and a carbonic acid diester in at least two reactors connected in series, wherein an end-capping agent is added through the inlet port of at least one reactor in which the limiting viscosity of the polymer has reached 0.20 dL/g.
As this patent literature discloses, practitioners of the art have experimented with adding end-capping agents when the polycarbonate reaches a specific limiting viscosity or a specific degree of polymerization. There still remains a need, however, for methods that will yield polycarbonates with the desired molecular weight, polycarbonates that do not decompose (by virtue of having too many terminal hydroxyl groups), and that provide improved end-capping efficiency.
Disclosed herein is a process for preparing an end-capped polycarbonate comprising melt reacting an aromatic dihydroxy compound, a carbonic acid diester, and optionally a catalyst in a prepolymerization vessel to form a polycarbonate product; transferring the polycarbonate product from the prepolymerization vessel; mixing the polycarbonate product with an end-capping agent under hermetic seal at a pressure of at least about 760 mm Hg; and transferring the polycarbonate product and end-capping agent mixture to a postpolymerization vessel to endcap the polycarbonate product.
Also disclosed is an apparatus for preparing an end-capped polycarbonate comprising a prepolymerization vessel having and inlet and an outlet; a postpolymerization vessel having an inlet and an outlet; a mixer in fluid communication with the outlet of the prepolymerization vessel and the inlet of the postpolymerization vessel; and a charging port through which an end-capping agent is charged to the mixer, wherein the mixer is hermetically sealed at greater than or equal to 760 mm Hg.